The present invention relates to a monaural and stereo audio signal control system for use in television signal distribution systems and, more particularly, to such audio control systems in cable television distribution systems.
Various systems have been developed for the distribution of television signals to viewers. These systems include conventional local-area radio frequency broadcast, distribution by cable, and wide-area satellite broadcast. In general, television signals that include content that is considered objectionable are subject to government regulation and/or industry self-regulation to prevent viewing by unauthorized persons, particularly children.
Conventional television signals include a video and an audio component; the audio component is presented via a modulated FM carrier for both monaural sound and/or stereo sound. In the earliest days of cable television, the cable TV provider distributed TV signals from a headend facility into the subscriber""s location via a set-top box. The cable television provider typically transmitted a number of channels greater than that provided by the area broadcast stations. The subscriber selected the desired channel which was then passed through the set-top box to the television receiver. The set-top box functioned essentially as a tuner for the many signals provided from the headend.
Later, with the advent of channels that carried objectionable content, the cable television provider scrambled the video component by changing the timing/position of the horizontal synchronization pulses or otherwise interfering with the video feed sent to the set-top box. The set-top box was provided with a subscriber processing section that enabled the authorized subscriber to control the set-top box to descramble the video signal for presentation to the television receiver. However, unauthorized viewers, e.g., children of the subscriber, could disconnect the cable from the set-top box and then reconnect the cable directly to the signal input of the television receiver and then tune the television receiver to the desired channel. While the scrambled video signal could not be viewed, the audio content was available. Oftentimes, the audio content was also considered objectionable when heard by children.
In order to prevent unauthorized users from listening to the audio component of the objectionable television signal, the cable television supplier also altered the audio component of the television signal. In the case of a monaural audio component, the carrier frequency of the FM audio signal was shifted. In the case of a stereo audio component, a second FM carrier with the audio component was added to the television signal. In either case, the audio component could not be demodulated within a conventional television receiver. Thus, disconnecting the cable from the set-top box and directly connecting the cable to the input of the television and tuning to an objectionable channel would cause the receiver to display the scrambled, unviewable video component without any meaningful audio component.
The set-top boxes were manufactured in accordance with the type of audio signal to be processed. During the time period in which monaural sound was dominant and stereo sound was only a small portion of the market, set-top box manufacturers would manufacture two separate product lines, one for monaural sound and the other for stereo sound.
FIG. 2A represents the audio processing path in a prior art monaural-only set-top box, and FIG. 2B represents the audio path in a prior art stereo-only set-top box. In both FIG. 2A and in FIG. 2B, the audio component of the television signal can be transmitted from the headend in either a xe2x80x98clearxe2x80x99 mode or a xe2x80x98maskedxe2x80x99 mode. In the clear mode, the audio component can be demodulated and processed by a conventional NTSC television receiver. In the masked mode, the audio component of the television signal is modified at the headend by the cable TV operator so that it is not demodulatable by a conventional TV receiver. The masked form of the signal has been commonly referred to as a xe2x80x9cprivacyxe2x80x9d mode.
In the monaural-only system of FIG. 2A, the audio component of the television signal from the headend can be transmitted to the set-top box in either the clear form or in the masked form. The clear signal conforms to the applicable signal standard (i.e., NTSC, PAL, etc.) in which the audio component is modulated on the sound carrier according to the signal standard or specification. The xe2x80x9cmaskedxe2x80x9d frequency corresponds to a frequency twice the horizontal sweep frequency (2H). The clear signal can be processed normally in a conventional TV receiver while the masked 2H signal cannot be properly demodulated and processed.
As shown in FIG. 2A, the monaural-only set-top box includes processing, designated by the reference character 10, designed to process the monaural audio associated with a clear signal and to process the monaural audio associated with a 2H masked audio signal.
The monaural processing system 10 includes a RF input 12 that provides the cable-supplied signals to a tuner 14, which, in turn, provides the selected signal to an intermediate frequency section 16. The output of the intermediate frequency section 16 is then provided to a down converter 18 and to a FM detector 20 that recovers the modulation envelope from the transmitted audio and presents that audio component to a clear signal path or a 2H masked path. The clear path includes an amplifier 22. The 2H path includes a 2H band-pass filter 24 that conveys the filtered signal to a detector 26, to a low-pass filter 28, and to an amplifier 30. The respective outputs of the clear path amplifier 22 and the masked path amplifier 30 are provided to a clear/masked select switch 32. The clear/masked select switch 32 is under the control of a microprocessor 34 that receives control information via a DATA input. The control information provided to the DATA input is provided by digital information impressed on the incoming video signal, typically on the portion of the retrace subsequent to the vertical synchronization pulse. The selected output of the clear/mask switch 32 is provided to an output amplifier 36 which provides its output to the audio-out connector 38 of the set-top box for connection to the subscriber""s television receiver.
During operation in which the signal from the headend has not been masked by the headend operator, the output of the FM detector 20 is provided through the amplifier 22 to the clear/masked select switch 32. The microprocessor 34, in response to control information provided on its DATA input, selects the clear signal for presentation through the amplifier 36 to the audio output connector 38.
During operation in which the signal from the headend has been masked, the output of the FM detector 20 is provided to the 2H band-pass filter 24 which conveys the filtered signal to the detector 26, to the low-pass filter 28, and to the amplifier 30. The output of the amplifier 30 is then presented to the clear/masked select switch 32. The microprocessor 34, in response to the control information provided to its DATA input, selects the output of the amplifier 30 for inputting to the amplifier 36 and presentation to the audio output connector 38.
In the event an unauthorized user, e.g., the children of the subscriber, attempts to circumvent the set-top box by disconnecting the cable input from the RF input 12 and connecting the cable directly to the television receiver, no intelligible audio will be provided when a masked audio component is being supplied by the headed operator.
The stereo system of FIG. 2B functions in a manner similar to that of the monaural system of FIG. 2A, but processes a clear or masked stereo signal in accordance with the BTSC (Broadcast Television System Committee) standard for multi-channel television sound (MTS). Under these standards, a xe2x80x9cmaskedxe2x80x9d stereo signal is transmitted on a second FM carrier.
As shown in FIG. 2B, the stereo-only set-top box includes processing, designated by the reference character 50, designed to process the stereo audio associated with a clear signal and to process the stereo audio associated with a xe2x80x9cmaskedxe2x80x9d second-carrier audio signal. The stereo processing system 50 includes a RF input 52 that provides the cable-supplied signals to a tuner 54, which, in turn, provides the selected signal to an intermediate frequency section 56. The output of the intermediate frequency section 56 is then provided to a down converter 58 for presentation to a xe2x80x9cclearxe2x80x9d stereo path or a xe2x80x9cmaskedxe2x80x9d stereo path. The xe2x80x9cclearxe2x80x9d stereo path includes a band-pass filter 60 that provides the filtered signal to an FM detector 62 with the demodulated output provided to an amplifier 64. The xe2x80x9cmaskedxe2x80x9d path stereo path includes a band-pass filter 68 that provides the filtered signal to a detector 70 that demodulates the audio information provided on the second carrier and provides that audio information through a low-pass filter 72 to an amplifier 74. The output of the amplifiers 64 and the amplifier 74 are provided to a clear/masked select switch 76 that is controlled by a microprocessor 78. The microprocessor 78 selects either the output of the amplifier 64 or the output of the amplifier 74 in response to control information presented to its DATA input with the selected signal provided via amplifier 80 to the output terminal 82.
During operation in which the stereo signal from the headend has not been masked by the headend operator, the output of the down converter 58 is provided through the band-pass filter 60 to the FM detector 62 and the amplifier 64 to the clear/masked select switch 76. The microprocessor 78, in response to control information provided on its DATA input, selects the output from the amplifier 64 for presentation through the amplifier 80 to the audio output connector 82.
During operation in which the signal from the headend has been masked, the output of the down converter 58 is provided through the band-pass filter 68 to the detector 70 and the low-pass filter 72 to the amplifier 74 to the clear/masked select switch 76. The microprocessor 78, in response to the control information provided to its DATA input, selects the output of the amplifier 74 for amplification by the amplifier 80 and presentation to the audio output connector 82.
In the event an unauthorized user, e.g., the children of the subscriber, attempts to circumvent the set-top box by disconnecting the cable input from the RF input 52 and connecting the cable directly to the television receiver, no intelligible audio will be provided when a masked stereo audio component is being supplied by the headend operator. A clear audio component will be demodulated and processed in the usual manner.
The prior art processing of FIGS. 2A and 2B was implemented in separate set-top boxes and supplied to the market in response to demand. Since stereo-capable systems have come to dominant the market, a need has arisen for a set-top box that is both monaural and stereo capable and able to function in response to clear and masked audio in each operating regime.
In view of the change in market demand from predominantly monaural to predominately stereo, the production of two separate set-top boxes, one monaural and the other stereo, is no longer justified from a competitive perspective.
In view of the above, it is an object of the present invention, among others, to provide a monaural and stereo audio signal control system for a cable television set-top box that automatically processes a monaural or stereo audio component of a television signal or a frequency-modified monaural or stereo audio component of a television signal.
An audio processing system for automatically processing both clear and masked audio signals of either the monaural or stereo type in accordance with the present invention includes a first processing path for clear monaural and clear stereo audio signals, a processing path for masked monaural audio signals, and a path for masked stereo audio signals.
The system includes a front end having a tuner, an intermediate frequency section, and a down-converter that processes all incoming audio signals. The down-converted signals are then presented to the several processing paths for processing in accordance with the type of audio signal received.
The monaural masked path subjects the signal to pre- and post-detector filtering and presents the detected and filtered output to a masked monaural/stereo selection switch. The masked stereo path likewise includes pre- and post-detector filtering and similarly presents its detected and filtered output to the masked monaural/stereo selection switch. A bandpass filter is provided in the masked stereo processing path and functions to detect the signal energy of those signal components that identifies a signal as a masked stereo signal. When this energy is detected, the masked monaural/stereo switch is appropriately actuated to route the masked stereo audio signal for further processing.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description to follow, taken in conjunction with the accompanying drawings, in which like parts are designated by like reference characters.